Industrial plants having containers or tanks (“tanks”) generally need to regularly measure the level of liquid(s) or other materials therein such as powders. There are several types of systems and techniques used for level measurement, which generally utilize time domain reflectometry (TDR) which relies on analyzing echoes.
For TDR-based measurements, there are contact level measurements, where a part of the system, such as a probe, contacts the material being measured, and non-contact level measurements where the level is measured with a probe without contacting the material to be measured. Non-contact methods include ultrasound which uses high-frequency sonic (sound) waves to detect the level, and radar which uses electromagnetic energy at radio frequencies which propagate through free-space.
Guided wave radar (GWR) is a particular contact pulsed radar method used to measure the level of liquids or solids in a tank. GWR works by generating a stream of pulses of electromagnetic energy and propagating the pulses down a transmission line formed into a level sensing probe, such as a coaxial probe. The probe is generally placed vertically in a tank or other container and the electromagnetic pulse is launched downward from the top of the probe. The probe is open to both the air and the material(s) to be sensed in such a way that the electromagnetic fields of the propagating pulse penetrate the air (or other gas, liquid or perhaps a solid) until they reach the level of the product material. At that point, the electromagnetic fields see the higher dielectric constant of the product material. This higher dielectric constant causes a reduction in the impedance of the transmission line, resulting in a pulse echo being reflected back to the top of the probe. The pulse travels through the generally air dielectric portion of the probe above the product material at a known velocity. Knowing both the return trip time of the radar pulse and the dielectric constant of the materials through which the radar pulse propagates allows the material level(s) on the probe to be determined.
An echo curve includes a plurality of different peaks, where each peak of the echo curve corresponds to a reflection of the radar signal at discontinuities caused by a transition between two media (e.g. from the surface of the level or interface, an obstacle, or something else). A reference pulse is one peak caused by the transition between transmitter head and the probe or an impedance transition inside the transmitter head. A reflection at the product surface(s) as described above causes further a further peak(s).
Besides their usual operating mode which is based on echo curves obtained from actual measurements, GWR systems generally also provide a simulation mode that can be used to simulate radar measurements and alarms. One application for echo curve simulation is when installing a pulsed radar level gauge on a tank for the first time or when troubleshooting an existing pulsed radar level gauge installation. In such applications, software-based field setup tools are typically used as a setup aid.
These set-up tools are used to program the pulsed radar level gauge with such input parameters as waveguide length, process connector characteristic(s), tank nozzle dimensions (for tanks having nozzles), product material(s) in the tank (their dielectric constants and expected tank disturbances (e.g., foam, turbulence). In addition to setting up the pulsed radar level gauge and application parameters, the set-up tool is generally also used to collect and display actual echo curves received responsive to transmitted radar pulses. Such displayed actual echo curves are then used by the operator (e.g., technician or engineer) to judge whether the pulsed radar level gauge has been set-up and is operating as expected.